1. Field of the Invention
The present invention generally relates to an echo canceller that can be used in a telecommunication network, for instance in a softphone, or a gateway. It peculiarly concerns an echo cancelling for a wide band audio signal. The present invention could be also applied for audio signals at sampling rate higher than 16000 Hz (examples: 22050 Hz, 32000 Hz, 44100 Hz . . . ).
The echo cancellation is required for audio communication over Internet protocol. The use of wide band audio implies the use of wide band echo cancellation. In order to achieve the same effectiveness as narrow band echo cancellation, in terms of quality, the wide band echo cancellation needs a greater computation volume, i.e. a greater number of millions of instructions per second (MIPS), by a factor four. It means that, if a narrow band echo cancellation requires n MIPS, a wide band echo cancellation using the same technology will require 4×n MIPS. In general, when the band width is doubled the computing volume of echo cancellation is quadrupled.
In a softphone, or a gateway, the echo cancelling is provided by an echo canceller constituted by a signal processor running echo cancelling software. As an audio codec used for wide band telephony requires more computation volume than an audio codec for narrow band telephony, the overall computation volume of a softphone is very high. In addition, a softphone usually runs with other applications at the same time in a personal computer. So it is important to reduce the computation volume of the echo canceller, while providing a high level of audio quality, peculiarly in hand free mode.
This invention deals with the problem of computation volume of echo cancellation, and proposes an echo canceller to reduce this volume without reducing the quality of the audio signal.
2. Description of the Prior Art
There are several known methods to reduce the computation volume of wide band echo cancellation. These methods are based on sub-band decomposition, or frequency domain computation. But these methods introduce:                An extra delay in the communication, which increases with the number of sub-bands. This delay is important for more than two sub-bands.        A degradation of audio quality (for more than two sub-bands). This degradation is due to an imperfect separation of the sub-bands (because no filter is ideal).        
There are also known methods avoiding these drawbacks. These methods are based on sub-band decomposition and sub-band filter adaptation. They consist in synthesizing a full band filter by using two sub-band filters. These methods are called delay less decomposition. The document WO 2007/021722 describes such a method. They are not good enough because:                They imply the use of Fast Fourier Transforms (FFT) and inverse Fast Fourier Transforms (iFFT) to synthesize the full band filter. They need extra computation because the computation of FFT and iFFT must be done with floating point coding of numbers.        They introduce a problem of quality, due to the synthesis of the full band filter, peculiarly with fixed point coding of numbers computing.        
Among known methods based on sub-band decomposition, decomposition in two sub-bands enables to reduce the computing by a factor of about 2 (less than 2 in fact). The document WO 2005/062595 describes such a method. For the wide band audio signal, this decomposition does not introduce a subjective degradation of the quality. But it would be desirable to reduce the computation volume more significantly.
FIG. 1 represents the functional diagram of an example EC1 of such a two sub-band echo canceller according to the prior art. This example comprises:                An input receiving a signal TXi from the near end terminal.        An output sending a signal TXo to the far end terminal, this signal being identical to the signal TXi received from the near end terminal.        An input receiving a signal RXi composed of a vocal signal captured by the far end terminal and an echo originating from the near end terminal and coming back via the far end terminal.        An output sending a signal RXo to the near end terminal. The echo is cancelled or at least attenuated in this signal Rxo.        A splitter device SP1 for separating the signal TXi into two sub-sampled signals TH and TL respectively corresponding to a higher sub-band and a lower sub-band.        Two adaptive filters AL and AH for respectively filtering the two signals TH and TL respectively corresponding to the higher sub-band and the lower sub-band. These filters supply filtered signals FTL and FTH respectively.        A splitter device SP2 for separating the signal RXi into two sub-sampled signals RH and RL respectively corresponding to the higher sub-band and the lower sub-band.        A first subtractor SH for subtracting the signal FTH to the signal RH, the resulting signal being a corrected signal CRH for the higher sub-band.        A second subtractor SL for subtracting the signal FTL to the signal RL, the resulting signal being a corrected signal CRL for the lower sub-band.        A mixer device MX for constituting the signal RXo by up-sampling, combining and smoothing the two corrected signals CRH and CRL.        
The two adaptive filters AL and AH are respectively controlled by the corrected signal CRL and CRH so that these corrected signals are minimized, i.e. so that the echo is minimized in each of the sub-bands. For instance, the higher sub-band spreads from 4000 Hz to 7000 HZ; and the lower sub-band spreads from 50 Hz to 4000 HZ.
The splitter device SP1 comprises a low-pass filter LP1 and a high pass filter HP1, associated respectively with two sub-sampling devices LSS1 and HSS1. One out of two successive samples of each of the two filtered signals, respectively supplied by the low-pass filter LP1 and the high pass filter HP1, is dropped for constituting respectively the signals TL and TH.
The splitter device SP2 comprises a low-pass filter LP2 and a high pass filter HP2, associated respectively with two sub-sampling devices LSS2 and HSS2 for generating respectively the signals RL and RH. One out of two successive samples of each of the two filtered signals, respectively supplied by the low-pass filter LP2 and the high pass filter HP2, is dropped for constituting respectively the signals RL and RH.
The mixer device MX comprises:                two up-samplers HUS and LUS for respectively up-sampling the corrected signals CRH and CRL, by duplicating each sample;        a high pass filter HP3 for filtering the signal restituted by the up-sampler HUS;        a low pass filter LP3 for filtering the signal restituted by the up-sampler LUS;        an adder device AO for adding the two filtered signals respectively supplied by the filters HP3 and LP3, and generating the output signal RXo to be supplied to the near end terminal.        
The computation volume for removing echo in each sub-band (in this example) is reduced by a factor four, due to a down-sampling factor equal to two. But there is an additional cost for decomposing the signals into two sub-bands and then synthesizing a complete signal. So the cost of this solution is:2×(Full Band cost)/4+Decomposition Cost+Synthesis Cost
The cost of this known solution is slightly greater than half the cost of a full band solution, because the decomposition cost and synthesis cost are very low with respect to the cost of echo filtering and adaptation. The same known method can be used for n band decomposition.
For the wide band audio signal, this decomposition does not introduce a subjective degradation of the quality. But it would be desirable to reduce the computation volume more significantly.
Thus, there is a need to provide a technical solution for further reducing the computing volume of the echo cancellation for wide band audio communication. This point is particularly important for mobile device and also multichannel echo canceller used on gateways. This can be solved by the echo canceller according to the invention.